Results from Phase 1 of this project indicated that transmission of S. mutans 6715 occurred among rats consuming 0, 0.1, 1 and 56 percent sucrose in their diets. Generally, increased levels of infection were associated with increases in dietary sucrose concentrations. Recipient rats fed 0-5 percent sucrose showed no difference in level of infection or any difference in smooth surface caries scores; those fed 56 percent sucrose attained the highest level of infection and also the highest caries of any group. Phase 2 of this project examined the transmission of S. mutans and subsequent caries development at intervals among rats consuming 56 percent sucrose in their diets. The data indicate that although the original inoculum for the donors was 10-8 to 10-9 cells/ml, their infection levels increased with time and appeared to plateau around day 40. Transmission from donors to recipients occurred as early as day 1 (1 pair) and was delayed as long as 17 days (3 pair). Transmission was detected in over 50 percent of the recipients by the 10th day and by day 17 approximately 90 percent of the recipients were infected. Means smooth surface caries scores appeared to be more strongly associated with length of infection rather than the infection level, i.e., regardless of the level of infection, mean caries scores increased with time.